This invention relates to flow control valves and, more particularly, to a needle valve used to control precise flow of a gas passing therethrough.
Needle valves are, of course, used in a wide variety of applications where precision is needed in varying flow over a particular range. The valves comprise a valve seat having a finely designed cylindrical bore and a tapered needle that moves within that cylindrical opening to vary the effective opening size and thus control the flow of gas through that opening. The movement of the needle is generally controlled by turning a valve stem which supports the needle and which is fitted into a housing with an appropriate threaded engagement.
One characteristic feature of such valves is a means to prevent backlash or movement of the valve stem with respect to its threaded housing as a result of play or clearance between the threads. Normally, a force is applied to maintain the threads of the valve stem against the corresponding threads of the housing, however, such force is applied in a non-uniform manner. That is, when the conventional needle valve is in its fully open position, i.e., the needle is as retracted from the cylindrical bore in its valve seat as is possible, there still must be a force on the valve stem holding it firmly against the respective threads. As the needle valve is then moved toward its closed position, that force is generally increased due to the means of applying the force, and thus such actual force increases to a high value that is really not necessary, causing unnecessary wear to the threads and shortening valve life.
Conventional valves further affix the needle itself to the valve stem such that the tapered end is held firmly in position with respect to the valve stem and, as a result, also with respect to the cylindrical opening in the valve seat. Accordingly, the tapered end, when rotated, will move not only laterally as is expected, but will also have some radial movement resulting from any eccentricities in the valve stem. The tapered end movement is therefore not precise within the valve seat and may vary in needle valves due to differences in manufacturing limitations.